1. Field of the Invention
The present invention relates to a method of heat-treating a piping system by heating a pipe with the use of high-frequency waves and a heating coil to be used for the heat treatment, and more particularly to a piping system heat-treating method and a heating coil which are suitable for the case in which the heating coil used has its shape such as its sectional shape abruptly changed as at a parent pipe and a weld zone of a pipe welding desk.
2. Description of the Prior Art
A piping used in nuclear, heat power or chemical plants is formed into a long piping system by jointing straight tubes, curved tubes or other tubes by the welding process. In recent years, however, since there is a tendency that a nuclear power plant, for example, is used at a high temperature and under a high pressure, there arises a fear that the stress to be generated in such piping may come close to the yield stress of a material used. On the other hand, the piping is plastically worked and welded to prepare a piping system. The tensile residual stress established in the piping as a result of those two processes is superposed upon a repeated stress generated during the runnihg operation of the plant such as a repeated thermal stress thereby to generate a high stress. Especially in case a corrosive fluid flows through a pipe as at a pipe of austenite stainless steel which is used in the piping of a boiling-water type nuclear power plant, it is necessary to take the corrosion fatigue into consideration, too. As a method for preventing such piping system from corroding and cracking due to stress, there has been proposed a piping system heat-treating method, as is disclosed in U.S. Pat. No. 4,229,235, for example, in which the outer wall of a pipe is heated, with a coolant existing in the pipe, to establish a temperature difference between the inner and outer walls of the pipe thereby to yield the inner wall to tension and the outer wall to compression. FIG. 1 shows the arrangement, in which a piping 1 having straight pipes 1A and 1B of an equal diameter arranged coaxially of each other has a high-frequency induction heating coil 3 arranged around a weld zone 2 thereof. In this heat-treating method, a coolant such as cooling water for a reactor in case the nuclear power plant, for example, is filled in the pipe, and a heating coil is wound on the outer wall of the pipe in the vicinity of the weld zone of the pipe. That coil is energized to generate a magnetic field. The pipe has its outer circumference heated but its inside temperature not elevated to a high level because of the existence of the coolant so that a temperature difference is established between the inner and outer walls of the pipe. This temperature desired is sufficient to yield the outer wall to compression and the inner wall to tension. After this heat treatment is finished in that way, the temperature of the pipe is gradually lowered so that a tensile residual stress is generated in the outer wall of the pipe whereas a compressive residual stress is generated in the inner wall of the pipe. Thanks to the generation of the compressive residual stress in the inner wall, the pipe has its corrosion fatigue strength remarkably augmented. In other words, the nuclear power plant is enabled to endure the repeated stress, which is generated as a result that the cooling water flows in the pipe during the running operation of the reactor, and to prevent the pipe from corroding and from being broken even if any corrosive fluid does exist.
There is a limit to the maximum temperature at which the plant piping to be heat-treated in the aforementioned manner can be heated. For example, in the case of the stainless steel used in the nuclear power plant, the maximum temperature at which the outer wall of the pipe can be heated is restricted to about 550.degree. C. because of a problem of sensitiveness resulting from the heat treatment. In order to generate the compressive yield stress in the outer wall of the pipe and the tensile yield stress in the inner wall within the limit of that range, it is an important subject matter how the large temperature difference required is to be ensured while suppressing the upper limit of the heating temperature. For the purpose of satisfying that subject matter, it is necessary to heat the respective portions of the outer pipe wall, which falls within a range to be influenced by the heating coil, as uniformly as possible without allowing any portion to have excessively high and low temperatures.
The uniform temperature distribution of the outer wall of the pipe when the heating coil is used can be easily obtained according to the theory in case the coil has a simple shape. In case the coil shape is complicated, however, the distribution of the magnetic flux to be generated by the action of the electric current flowing through the coil is changed to make the temperature distribution irregular.
This irregularity in the temperature distribution is also caused by the complex coil shape in the vicinity of the weld zone of a pipe welding desk. This corresponds to the case, in which the pipe welding desk is welded to the parent pipe so that a branch pipe may be connected at a right angle to the parent pipe. In this case, the pipe shape itself becomes complex so that the coil shape is accordingly complicated. In this case, there is left a portion which is reluctant to have its temperature raised even in accordance with the change in the distribution of the magnetic flux.
A desk fitting-up weld zone is located at the boundary between the parent pipe and the pipe welding desk, and this desk has a smaller diameter than that of the parent pipe. In a recirculating piping of the nuclear power plant, for example, the parent pipe has a diameter of 28 inches, and the desk has a diameter of 4 inches and is a short pipe which is directed substantially at a right angle with respect to the parent pipe.
According to the prior art, as shown in FIGS. 2 and 3, in case the residual stress of a desk fitting-up weld zone M is to be improved, a high-frequency induction heating coil 4 is wound around a parent pipe 5 and is arranged along the circumferential side portion of a pipe welding desk 6 at the fitting portion of the desk 6. FIG. 3 is a section taken along line X--X of FIG. 2.
In this case, the high-frequency induction heating coil 4 to be arranged along the circumferential side portion of the desk 6 covers the circumferential side of the desk 6, which is in the same direction as the winding one of the aforementioned high-frequency induction heating coil 4, so that it fails to sufficiently cover the circumferential side portion which intersects the winding direction of the aforementioned high-frequency induction heating coil 4. As a result, the region P of the desk 6, as shown, becomes that which is not heated so that the a weld zone N and the weld zone M fail to have uniform temperature distributions.
In this case, moreover, it is anticipated that a complex stress due to a local heat treatment is generated in association with the solution heat treatment, water-cooled welding treatment or buttering treatment, which is adopted as a counter-measure for preventing the corrosion and crack according to the prior art. It is feared to have an adverse effect, as the case may be.